This invention relates to the deposition of thin films, and, more particularly, to sources for cluster beams.
The deposition of thin films upon substrates is an important manufacturing and research tool in a variety of fields. For example, microelectronic devices are prepared by depositing successive film layers onto a substrate to obtain specific electronic properties of the composite. Photosensitive devices such as vidicons and solar cells are manufactured by depositing films of photosensitive materials onto substrates. Optical properties of lenses are improved by depositing films onto their surfaces. These examples are, of course, only illustrative of the thousands of applications of thin film deposition techniques.
In the highly controlled approach to thin-film deposition that is characteristic of applications where a high quality film is required, the film is built up by successive deposition of monolayers of the film, each layer being one atom thick. The mechanics of the deposition process can best be considered in atomistic terms. Generally, in such a process the surface of the substrate must be carefully cleaned, since minor contaminant masses or even contaminant atoms can significantly impede the deposition of the required highly perfect film. The material of the film is then deposited by one of many techniques developed for various applications, such as vapor deposition, electron beam evaporation, sputtering, or chemical vapor deposition, to name a few.
In another technique for depositing thin films, ionized clusters of atoms are formed in a cluster deposition apparatus. These clusters usually have on the order of about 1000-2000 atoms each. The clusters are ionized and then accelerated toward the substrate target by an electrical potential that imparts an energy to the cluster equal to the accelerating voltage times the ionization level of the cluster. Upon reaching the surface of the substrate, the clusters disintegrate at impact into atoms free to move on the surface. Each atom fragment remaining after disintegration has an energy equal to the total energy of the cluster divided by the number of atoms in the cluster. The cluster prior to disintegration therefore has a relatively high mass and energy, while each atom remaining after disintegration has a relatively low mass and energy. The energy of the atom deposited upon the surface gives it mobility on the surface, so that it can move to kinks or holes that might be present on the surface. The deposited atom comes to rest in the imperfections, thereby removing the imperfection and increasing the perfection and density of the film. Other approaches to using clusters have been developed, and thin film deposition using cluster beams is a promising commercial film manufacturing technique.
The cluster source, which produces the clusters, is a key component of a cluster beam deposition apparatus. The cluster source should produce a high mass flow of clusters of a selected size range, and exhibit a high cluster-forming efficiency. That is, the cluster beam should have a large fraction of the mass of the beam present as clusters rather than atoms, or the beneficial effect of using clusters may be lost. The cluster source should also provide a cluster beam with the clusters in the proper energetic state.
One type of cluster source is the carrier-gas cluster source, wherein a stream of the atoms to be condensed into clusters is emitted from a crucible into a gas mixing chamber. In the gas mixing chamber, a carrier gas is mixed with the stream of atoms, quenching the atoms to supersaturation and forming clusters. The clusters emerging from the source pass into a vacuum and are ionized and accelerated toward the target.
Carrier-gas cluster sources have two significant advantages over surface-growth cluster sources, the other most significant type of source. The carrier-gas cluster sources have a much higher efficiency of cluster formation, resulting in a higher fraction of the mass of the cluster beam being clusters rather than atoms. Second, the carrier-gas cluster sources can be used to form cluster beams of materials with extremely high melting points, such as refractory metals, which is not possible with surface-growth cluster sources.
However, it has been observed experimentally that the films deposited using conventional carrier-gas cluster sources tend to be grainy and rough. As a result, the films are not useful for many types of demanding applications such as microelectronic devices. It would be desirable to improve the quality of the deposited films obtained from carrier-gas cluster sources, while retaining the advantages in efficiency and versatility presently enjoyed by such sources. The present invention fulfills this need, and further provides related advantages.